A vast collection of man-made and plant-derived chemicals that function as insecticides has been amassed over the past sixty years. Concerns over the use of these insecticides, the development of insect resistance, and the possible risk of long-term use for human health have fueled efforts to understand the mechanism by which such compounds act. Some, like the organo-chlorine DTT, inhibit ATP production (Sacklin, J. A. et al., 1955, Science, 122:377-378). Others such as pyrethrins and organophosphates are neurotoxic (Soderlund, D. M., et al., 2002, Toxicology, 171:3-59; Johnson, M. K.,. 1975, Arch. Toxicol., 34:259-288).
Insect development appears to be driven by the action of at least two hormone classes, the ecdysteroids and the juvenile hormones (JHs, juvenoids) (Riddiford, L. M., 1994, Adv. Insect Physiol., 24:213-274; Gilbert, L. I., et al., 2000, Insect Biochem. Mol. Biol., 30:617-644; Thummel, C. S., 2002, Insect Biochem. Mol. Biol., 32:113-120). It appears that ecdysteroids are responsible for initiating metamorphosis, and in some insects, regulating adult fertility. In contrast, JH appears to be required for reproductive processes such as adult female vitellogenesis (Wyatt, G. R., and K. G. Davey, 1996, Adv. Insect Physiol., 26:1-15). Also, the simultaneous presence of ecdysteroids and juvenile hormone (JH) leads to larval-larval molting.
There are two heterodimeric partners that comprise the functional insect ecdysteroid receptor complex: the ecdysone receptor (EcR) (Koelle et al, 1991, Cell, 67:59-77) and Ultraspiracle (USP) (Oro, A. E., et al., 1990, Nature, 347:298-301; Henrich, V. C., et al., 1994, Dev. Biol., 165:38-52). Both EcR and USP belong to the nuclear receptor superfamily, which includes receptors for steroid and thryroid hormones, retinoic acid, and fatty acids (Mangelsdorf et al., 1995, Cell, 83:835-839). Also, recently, a second receptor, DHR38, as a heterodimeric partner of USP has been shown to mediate ecdysteroid responses, but the mode of action is not classical, in that it does not involve direct binding of the ecdysteroid to either DHR38 or USP (Baker et al, 2003, Cell 113:731-742).
There may also be mammalian counterparts to the insect receptor for ectdysteroids. Thus, EcR structurally resembles the vertebrate famesoid X-activated receptor (FXR) (Forman, B. M., et al., 1995, Cell 81:687-695). FXR is a member of the steroid receptor family that includes receptors for glucocorticoids, estrogen, vitamins A and D, thyroid hormones, and fatty acids. Also, there is evidence to suggest that USP may be the insect orthologue of the vertebrate retinoid X receptor (RXR) (Oro, A. E., et al., 1990, Nature 347:298-301). Comparisons of amino acids in the FXR and EcR DNA-binding domains reveal 60% identity, and the ligand binding domains (LBD) regions of these two receptors share about 45% identity. Nevertheless, the possible functional analogy between FXR/RXR and EcR/USP has yet to be resolved. For example, it has been shown that the vertebrate RXR is activated by methoprene acid but not JHIII or methoprene (Harmon, M. A., et al., 1995, Proc. Natl. Acad. Sci. USA, 92:6157-6160). In contrast, the insect USP complexes with JHIII and methoprene but not their acid metabolites (Jones, G. et al., 1997, Proc. Natl. Acad. Sci. USA, 94:13499-13503).
Due to concerns about possible toxicity of man-made insecticides, there is a need to identify natural compounds that have the ability to modulate insect survival and development. Understanding the basis by which compounds interact with the insect EcR and/or FXR to interfere with insect development may provide a rational basis for the isolation of safe, but effective insecticides.